A variety of materials such as laminates have been previously proposed for heating foods with microwave energy by absorbing a portion of the microwave energy and transmitting it in the form of heat by conduction to an object such as a food product. In some cases sheet material of this kind is stiff, brittle, subject to breakage and is not adapted to use in lightweight packaging products which should be disposable and low in cost. In other cases the laminates, while interacting with the microwave energy present in an oven, do not adequately heat the food product. Still other laminates can heat only one side of the food product. So, for example, if the food product is rectangular in shape, three sides remain unheated.
In view of the deficiencies of the prior art, it is one object to provide microwave interactive sheet material which furnishes compartments to loosely enclose food and particularly food in stick form, e.g., fish sticks or french fried potatoes and the like, and to heat the food on all sides. The application of heat to all sides is highly beneficial because it has been found that when a food piece such as a french fried potato is placed in an ordinary paper carton and heated in a microwave oven, the potato becomes soggy. This occurs even if an effort is made to allow steam vent openings at the top of the package. As a result, attempts have been made to develop laminates for lining food cartons to augment the heat provided by direct microwave interaction with the food. For example, U.S. Pat. No. 4,612,431 and the Assignees' copending application Ser. No. 740,252, now U.S. Pat. No. 4,735,513, describe laminates comprising polyester to which thin, semiconductive layers of metal have been applied. These laminates are bonded to one inside wall of the package for absorbing microwave energy and then transfering the energy to the food product. Tests conducted by us show, however, that these laminates and the resulting packages are not effective in crisping, browning or toasting the surface of foods such as french fried potatoes. After heating, the products are perceived to be moist, limp and soggy. A major objective of the invention is therefore to provide a microwave interactive laminate that will crisp, toast or brown several surfaces of a stick-shaped food product such as french fried potatoes, fish sticks and the like so that after heating it is perceived to be crisp and appetizing to the consumer.
Other attempts have been made to deal with this problem. For example, U.S. Pat. Nos. 4,267,420 and 4,230,924 provide a lightweight flexible wrapper formed from a laminate composed of a flexible sheet material such as metallized plastic film supported by a paper backing that interacts with microwave energy. One major shortcoming is that the food sticks have to be individually wrapped and later unwrapped one-by-one by the customer. Another problem results from the fact that portions of the sheet material can shrivel, shrink, split and crack, particularly in areas where it is not in contact with the food.